1. Field of the Invention
The present invention relates to flat display devices generally, and more particularly, to a flat display device that includes barriers to prevent ink from overflowing to neighboring pixel regions during an inkjet print process, and a method of manufacturing the same.
2. Description of the Related Art
An inkjet printing process is a significant structuring process for the production of a full-color image formed by a semi-conducting light-emitting polymer (LEP). In this case, small drops of a solution that include the corresponding polymer are deposited onto a suitable substrate. This process may also be used in other technical areas such as the deposition of color filters or DNA sensors onto a substrate.
All of these applications demand an exact placement of the substances (ink) deposited onto a predetermined active surface. The inkjet printing technique has this property. In inkjet printing, ink is produced by dissolving an active substance in an auxiliary substance. Then, this ink is deposited in small quantities in drop form onto the substrate, e.g., by means of a piezo or “bubble jet” inkjet technique. The exact positioning of the drop on the substrate is accomplished mechanically positioning the inkjet head over the substrate. After evaporation of the auxiliary substance, the active substance forms a film on the active surface of the substrate.
A frequent problem during the printing process is the dispersion of a drop of ink from the active substance into neighbouring surfaces of the substrate. This results in a mixing of colors in display elements including organic light-emitting diodes (OLED) because red, green or blue emitting areas are arranged in the immediate vicinity of one another.
Developed in the late eighties of the last century. OLED displays can be categorized into polymer OLEDs (PLED) and low-molecular OLEDs (SM-OLED). For example, PCT Patent No. WO 00/76008A1 (CDT) discloses the structure of a PLED display element. Additionally, Patent Nos. U.S. Pat. No. 4,539,507 and U.S. Pat. No. 4,885,211 (Eastman-Kodak) disclose the principle structure of an SM-OLED in which ALQ3 (tris-(5-chloro-8-hydroxy-quinolinato)-aluminium) is used as a light-emitting and electron transporting material.
The fundamental principle upon which OLEDs are based is electro luminescence, whereby electrons and holes are injected into a semi-conducting material via suitable contacts, and light is generated by the recombination of these charge carriers.
A piezo inkjet printing technique is one of the most significant structuring technologies for the production of full-colur displays based on polymer OLEDs. Small drops of a solution containing the active substance (hole transporting or light-emitting material) are deposited on the active surface of a suitable substrate. The dimension of the active surface (single pixel) for a high-resolution display element, as used, for example, in modern mobile telephones, is in the range of 40 μm×180 μm.
Conventional inkjet heads can produce ink drops with a diameter of 30 μm. The ink drop has a diameter of the same magnitude as a pixel to be coated. In order to prevent the drop from overflowing, the surface of the substrate is formed by suitable means.
In this regard, two strategies are pursued:
A substrate surface can be produced in such a way that different areas have different surface energies and therefore allow the ink to move in different ways. Also, geometrical (mechanical) barriers designed to prevent an overflow of the drop may be used.
An application of the first strategy is described in European Patent No. 0989778 A1 (Seiko-Epson). A non-constant surface energy is created by a suitable selection of materials which form the substrate surface. The ink can only spread in areas with high surface energy whereas areas with low surface energy act as barriers. In order to obtain a homogenous thickness of the film, it is also an advantage to form a high surface energy region beyond the periphery of the pixel surface of the OLED. The film is then homogenous in the peripheral zone and the layer thickness noticeably reduces just outside of the active zone near the barrier. The required differences between the surface energies can be achieved in many different ways. European Patent No. 0989778 A1 (Seiko Epson) discloses a substrate surface having a two-layer structure. By means of a suitable surface treatment using plasma, the upper layer of the substrate can be provided with low surface energy while the lower layer, based on its chemical nature, can be provided with high surface energy using the same treatment. Conventionally, the lower layer is composed of inorganic materials such as silicon oxide/nitride.
In this example, the inorganic layer acts as a peripheral zone with high surface energy and facilitates the depositing of homogenous polymer films using the inkjet printing process.
The depositing and structuring of this layer, however, requires processes used typically in the semi-conductor industry. For the layer deposition, sputter processes and gas phase processes such as PECVD (Plasma Enhanced Chemical Vapour Deposition) can be used. These processes require long pulse times and, in addition, are cost-intensive, thus reducing the cost-advantage gained with the OLED-technology. In addition, the second layer provides a surface topography, that is, the areas with low surface energy (called “separators” here) have a finite height above the main substrate surface. As a result of this height profile, the separated polymer film can form an undesirable thickness profile when curving upwards into the peripheral areas at the separators. Depending on the dimensions, the upward curving can protrude into the pixels.
A further disadvantage, discussed in EP 0989778, is that an ink reservoir is used as further overflow protection. This reservoir must have at least a minimum volume in order to accommodate a sufficient partial quantity of an (ink-) drop. A necessary depth of several 100 nm produces an edge upon which the subsequently deposited cathode layer can break. This can lead to an increased power input or a failure of an indicating element. The structuring of this reservoir is time-consuming and increases the technological difficulty due to the incorporation of a further process step.
Japanese laid-open patent No. 09203803 discloses a chemical treatment of a substrate surface that had been treated with a photo-resist beforehand. Following this, the photo-resist is exposed while using a mask, and developed. In the resulting structure, the areas where the photo-resist remains have low surface energy while areas where the photo-resist are removed have high surface energy. Flanks of the photo-resist structure have a mean surface energy and, thus, there is a fairly smooth transition of the surface energies. However, the flanks do not form a boundary with a freely selectable surface energy and geometry. This is a disadvantage because the spatial dissolution capacity of the inkjet printing process declines in areas with the mean surface energy. A further disadvantage is the fact that only one photo-resist composed of a single material can be used. Therefore, a contrast of the surface energies cannot be produced by the application of various materials, and this restricts the applicability of the OLED. In addition, the chemical treatment required is time-consuming resulting in a long manufacturing time.
Japanese Laid-open Patent No. 09230129 discloses a two-stage treatment of a surface. First, the entire surface is provided with low surface energy. As a result of a subsequent treatment of selected regions of the surface with short-wave length light, the surface energy is these areas is increased. The contrast in the surface energy is, however, limited, and the required exposure time is too long a mass production.
Geometrical (mechanical) barriers are described as a second possibility for preventing an overflow of a drop of ink.
U.S. Pat. No. 6,388,377 B1 discloses photo-resist stripe structures which are positioned between neighbouring pixels. These photo-resist stripes have a height of at least 2 μm and form a physical barrier preventing an overflow of an ink drop. The production of such a photo-resist structure is disclosed in European Patent No. 0996314 A1. Two photo-resist structures in each case, arranged parallel to one another form banks of a channel, in the centre of which there are pixels which later emit the same colour (red, green or blue). The printing of a suitable ink in the channel provides the pixels with active material and the photo-resist structure prevents an overflow of ink to pixels adjacent to the channel. The height of the banks is larger than 0.5×(width of the pixel/diameter of the drop). The height is also greater than the thickness of the active material deposited by means of the inkjet printing technique. Banks with a fine structure are obtained by applying round, oval or triangular notch indentations to the banks thus forming an overflow reservoir. However, the height of the banks and/or the edges reduces the quality of metal deposition in a subsequent step. In the metal deposition, the cathode of the OLED structural element is formed by thermal evaporation or sputtering. Based on the form and height of the photo-resist structures an interruption occurs, or at least a thinner deposition of the metal film on the side walls of the banks is obtained. This leads to increased electrical resistance which has a disadvantageous effect on the power input of the display element.